It is known from Canadian Pat. No. 913,328 to produce chlorine dioxide by the reduction of sodium chlorate with chloride ions in an acid aqueous reaction medium in which the chloride ions and the acidity are introduced by hydrochloric acid.
This prior art procedure may be carried out in a continuous manner with the reaction medium being maintained at its boiling point at the absolute pressure thereon while the reaction zone is maintained under a subatmospheric pressure, typically in the range of about 20 to about 400 mm Hg. The boiling temperature, which is maintained below that above which substantial decomposition of chlorine dioxide occurs, results in the evaporation of water from the reaction medium, the evaporated water acting as the diluent gas for the chlorine dioxide and chlorine. Sufficient water is evaporated from the reaction medium to balance the water introduced with the reactants and formed in the reaction medium, so that the liquid level in the reaction zone is maintained substantially constant.
After start up, the reaction medium becomes saturated with by-product sodium chloride and this sodium chloride precipitates from the reaction medium. Under the steady state conditions of the continuous operation, sodium chloride continues to precipitate as the reaction progresses and is removed, continuously or intermittently, from the reaction zone.
Two competing reactions occur in the reaction medium, in accordance with the following equations (1) and (2): EQU NaClO.sub.3 + 2HCl .fwdarw. ClO.sub.2 +1/2Cl.sub.2 + H.sub.2 O + NaCl (1) EQU NaClO.sub.3 + 6HCl .fwdarw. 3Cl.sub.2 + 3H.sub.2 O + NaCl (2)
The efficiency of conversion of sodium chlorate to chlorine dioxide is determined by the extent to which reaction according to equation (1) predominates over reaction according to equation (2), since no chlorine dioxide is formed by the latter reaction. The production of chlorine dioxide at high efficiency depends therefore on the extent to which reaction according to equation (1) can be made to predominate over reaction according to equation (2).